On price, the plain 645 will do the job. Considerations such as Tom's point to the 645N - and I'll add you get 16 shots per 120 roll with the n, and 15 with the 645.
Either camera will work well hand held - they are deeper than 35mm SLRs, but the handle is good for your right hand, and you can steady the camera underneath and on the lens with your left. These are pretty well balanced and the center of gravity is good, so stable hand held shooting is practical. The shutter/mirror/film advance motor is NOISY though